PLANKTON PRODUCTION IN FISH PONDS 
139 
ligently, we must first of all have some idea as to what elements necessary to photo- 
synthesis are becoming exhausted. In other words, which are the limiting factors? 
In an attempt to answer this question, the following chemical determinations were 
made quantitatively: Organic nitrogen, nitrate nitrogen, nitrite nitrogen, ammonia 
nitrogen, dissolved phosphorus, organic phosphorus, chloride, free carbon dioxide, 
phenolphthalein alkalinity, pH, and dissolved oxygen. It is important that all the 
four forms of nitrogen and the two forms of phosphorus be determined, because an 
element must not only be present but it must be in an available form. In the case 
of nitrogen, for instance, the nitrate alone is immediately available to a majority of 
algae. Some of the blue-green appear to utilize compounds of ammonia, but the 
nitrate seems to be preferred by most. The organic nitrogen and the nitrite nitrogen 
are not immediately available to any of the algae. Hence, to determine total nitro- 
gen or even total inorganic nitrogen would be misleading. The same is also true of 
the phosphorus, for it is only the dissolved phosphorus that is immediately available 
for plant growth. A major object of these chemical determinations was to find out 
if the inorganic nitrogen or the dissolved phosphorus ever became completely ex- 
hausted. The data presented in this report show that nitrate and ammonia nitrogen 
were always present, even in the unfertilized ponds. The dissolved phosphorus, 
however, becomes at times completely exhausted. 
The determinations of pH and phenolphthalein alkalinity were made for the 
purpose of finding out whether or not the hydrogen-ion concentration may be a 
controlling factor in the growth of plankton. The results seem to show that within 
fairly wide limits the hydrogen-ion is not the controlling factor. The high values 
for alkalinity and for pH seem to be the direct result of photosynthesis. In fact, it 
looks very much as if the rate of photosynthesis controls alkalinity rather than the 
reverse. Another reason for making these determinations was to see just how great 
these variations really are. 
That pH and alkalinity in these ponds would be governed very largely by the 
rate of photosynthesis would probably be expected, for carbonic acid is undoubtedly 
the chief acid in these pond waters. Now photosynthesis uses not only the free 
carbonic acid but some of that in loose combination with the metals calcium and 
magnesium. The withdrawal of carbonic acid would tend to make the water alkaline. 
The determinations of free C0 2 were made for several reasons. In the first 
place, we wanted to know how close the correlation is between pH and C0 2 . If the 
hydrogen-ion concentration in these pond waters is due largely to C0 2 , then the pH 
values and the values for C0 2 should be in an inverse ratio ; that is, as the C0 2 goes 
up the pH should go down. This assumption is borne out by the results. Another 
reason for making determinations of free C0 2 was to see if this acid might ever be 
present in sufficiently large quantities to become detrimental to fish fife. Still a 
third reason is that since C0 2 is one of the raw materials for photosynthesis it may 
become a limiting factor. 
The dissolved oxygen determinations are important for two reasons. In the 
first place, it seemed worth while to determine just how abundant this element is 
and to what extent it varies in amount. Another reason was to see if the dissolved 
oxygen would ever become low enough to endanger fish life. This was of especial 
importance in those cases where organic fertilizers were added to the water. 
Apparently the amount of fertilizers used in our pond work did not seriously affect 
the oxygen supply. 
